Although I've been extremely happy with my 26 preamp/DAC, the tweaking bug bit me again so I decided to try out some 01A tubes in place of the 26.
I picked up a nice NOS pair of CX-301A online and set out to make the modification.
It turned out to be much easier than I though it would be, as no modifications were needed to any of the power supplies or even to the biasing arrangement. The 26s were biased with 9V lithium batteries on the grid that actually put out 9.5V with no load (fed through the LL1676 input transformer secondary, so no DC blocking cap needed) and this turned out to be just right to bias the 01A at ~-4.8V given I am running the 01A filament at +4.7V using Coleman regulators. All I needed to do was adjust the plate CCS to 3mA, change R1 in the Coleman regulators to 4.4ohm from 1 ohm, and adjust the Coleman regulators. I soldered a pc-mount terminal block onto the Colemans to make resistor swapping easier in case the bug bit me again (picture 1). The output is taken from the mu output of the CCS to a Bent Audio remote controlled Slagleformer volume control in a lossy parafeed arrangement - the volume control has ~ 150H inductance. Silver wire on a nickel core. No problems driving anything to date.
A few pictures. The preamp as first built (picture 2). Topless shot with the new CX-301A tubes (picture 3). The Buffalo IIISE DAC and Sonore USB input boards are in the top right corner of the box. I have a switch on the back to go from DAC (SPDIF or USB) to preamp (5 line inputs).
So...how does it sound? I've been a staunch supporter of the 26 as a preamp tube, but my first impressions are that the 01A has it beat in terms of detail, air and that certain lush sound, without losing any of the punch or low end. Snare drum shots and massed horns are startling in their intensity. The 26 sounds a bit less precise and a little less "juicy" in comparison. More listening to come but I'm thinking it's going to be hard to go back to the 26 for quite a while...I may also need to get a new faceplate...Max01A maybe?? Or maybe I'll be non-committal and go with MaxDHT.
PS: Rod Coleman was his usual extremely helpful self when I peppered him with questions about his regs and the 01A. Thanks, Rod!
I picked up a nice NOS pair of CX-301A online and set out to make the modification.
It turned out to be much easier than I though it would be, as no modifications were needed to any of the power supplies or even to the biasing arrangement. The 26s were biased with 9V lithium batteries on the grid that actually put out 9.5V with no load (fed through the LL1676 input transformer secondary, so no DC blocking cap needed) and this turned out to be just right to bias the 01A at ~-4.8V given I am running the 01A filament at +4.7V using Coleman regulators. All I needed to do was adjust the plate CCS to 3mA, change R1 in the Coleman regulators to 4.4ohm from 1 ohm, and adjust the Coleman regulators. I soldered a pc-mount terminal block onto the Colemans to make resistor swapping easier in case the bug bit me again (picture 1). The output is taken from the mu output of the CCS to a Bent Audio remote controlled Slagleformer volume control in a lossy parafeed arrangement - the volume control has ~ 150H inductance. Silver wire on a nickel core. No problems driving anything to date.
A few pictures. The preamp as first built (picture 2). Topless shot with the new CX-301A tubes (picture 3). The Buffalo IIISE DAC and Sonore USB input boards are in the top right corner of the box. I have a switch on the back to go from DAC (SPDIF or USB) to preamp (5 line inputs).
So...how does it sound? I've been a staunch supporter of the 26 as a preamp tube, but my first impressions are that the 01A has it beat in terms of detail, air and that certain lush sound, without losing any of the punch or low end. Snare drum shots and massed horns are startling in their intensity. The 26 sounds a bit less precise and a little less "juicy" in comparison. More listening to come but I'm thinking it's going to be hard to go back to the 26 for quite a while...I may also need to get a new faceplate...Max01A maybe?? Or maybe I'll be non-committal and go with MaxDHT.
PS: Rod Coleman was his usual extremely helpful self when I peppered him with questions about his regs and the 01A. Thanks, Rod!
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Thanks, Sumotan! Listening to the 01As now with a little Grant Green. Green Street sounds very nice through them; the pluck of the guitar and the bass lines are very well defined. It's not a night and day difference as it was when I went from 6SN7/2C22 to the 26, but I think there's a little more information coming through now. I'm thinking I'm a fan of thoriated filaments vs oxide-coated - now with the 01As and the 833Cs in my amps I get a double dose of thorium!
OMG that's a gigantic tube that's what scares me away the very high voltage involved.
In any case kudos high quality stuff that you've build.
Take care
In any case kudos high quality stuff that you've build.
Take care
It scared me when I built it, too...first turn-on was with a long wooden stick pushing the high voltage button while holding a fire extinguisher in the other hand. That was 7 years ago I guess and now I'm not so scared anymore, but still respectful. ;-)
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801 as a preamp tube? A bit of overkill, I think. Nice power tube, but I have the 833 to satisfy my amp thorium habit. ;-)
801 as a preamp tube? A bit of overkill, I think. Nice power tube, but I have the 833 to satisfy my amp thorium habit. ;-)
Here’s a basic 801 preamp schema posted by Thomas Mayer some years ago. Has a Lundahl line OPT (step down) with Slagle AVC at the output for volume control..
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And how would you describe the 801 vs the 01A as a preamp? What advantages are there to using the 801?
I'm using a remote control Slagleformer on the preamp output, but instead of an additional OPT before it I just used a cap tuned with the autoformer in parafeed. In effect, the autoformer is the OPT. Works great!
A few years (more than a decade) ago my daily used preamp was CCS loaded #26 (filament biased with R.C. regulator), capacitor fed to S&B TX-102 TVC.
I was satisfied with it, the globe #26 tubes produced flattering sound, especially the middle range was super.
The CCS "low Z" output was enough to driving TVC.
I'm also a fan of TT filament "sound", so it was a logical step to try #01a ... and there was a little bad surprise: in the same structure the #01a driving capacity wasn't enough to properly cooperate with TVC (400H inductance!).
I tried it with Ale's gyrator too, measure its for days, but without FET source follower it didn't give a satisfactory result comparing to #26. The HF region was problematic, not so good as -in the same preamp- the #26 tube.
Meanwhile I tested my breadboarded 801a PSE amp (841, 10Y, 801a PSE), so I threw together a simple gyrator loaded, filament biased 801 preamp, which "tone" was far better, than #26 or #01a preamp.
It became the final (? 😛pp) choice:
Input selector, NOS TKD potentiometer, filament biased -graphite anode 801 (not 801a!), gyrator load, capacitor coupled output.
The next step would be output to AVC integration (I have spare Silk and Promitheus nano core ones too), but .... it sounds perfectly -for me-.
Now the #26 preamp singing in my "second" music equipment.
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Interesting, Euro. I haven't heard any issues so far with the 01A driving my TVC, at either 4mA or 3mA plate current. I'll keep the 801 in mind, though!
After some testing today, I've decided to go with 4mA plate current on the 01A. The bass is just a bit more defined than at 3mA, which makes sense since the lossy parafeed output was optimized for the 8K plate resistance of the 26. According to the CX-301A datasheet, the 01A is about 10K at 3mA and close to 8K at 4mA.
Cool. I think I'll stick with the 01A for a while as I don't need the increased drive of the 801. My next step is to replace the diaphragms in the 6 EMIT tweeters with new replacements; the originals from 1985 are starting to show their age with a few wrinkles in the kaptan. The EMIM midranges have already been upgraded with new replacement diaphragms from Apogee Acoustics.
One more observation about the 01A vs 26. With the 26, images of a singer's voice were always high on the front wall, nearer to the ceiling. With the 01A, the images are right at my eye level, centered between the ceiling and floor; I've listened to 3 different sets of both CX-301A and UX-201A and all of them do the same thing. I like it, it's more realistic to me.
I haven't made any changes to the room or my electronics other than the tubes, so it must be the tubes making the difference.
I haven't made any changes to the room or my electronics other than the tubes, so it must be the tubes making the difference.